US subs remain far better than their Chinese counterparts, but in a conflict, numbers and geography may help China mitigate some of the US and its partners’ advantages.

Naval modernization is part of Beijing’s “growing emphasis on the maritime domain,” the US Defense Department said in its annual report on Chinese military power.

As operational demands on China’s People’s Liberation Army Navy have increased, subs have become a high priority — and one that could counter the US Navy’s mastery of the sea.

Chinese sailors salute from a submarine during a China-Russia joint naval exercise in the Yellow Sea, April 26, 2012. REUTERS/China Daily

The force currently numbers 56 subs — four nuclear-powered missile subs, five nuclear-powered attack subs, and 47 diesel-powered attack subs — and is likely grow to between 69 and 78 subs by 2020, according to the Pentagon.

China has built 10 nuclear-powered subs over the past 15 years. Its four operational Jin-class missile boats “represent China’s first credible, seabased nuclear deterrent,” the Pentagon report said.

In most likely conflict scenarios, however, those nuclear-powered subs would have limited utility, said Bryan Clark, a senior fellow at the Center for Budgetary and Strategic Assessments.

“They’re relatively loud, pretty easy to track, and don’t really have significant capability other than they can launch land-attack cruise missiles, and they don’t have very many of those,” Clark said. “They’re more of a kind of threat the Chinese might use to maybe do an attack on a … more distant target like Guam or Hawaii.”

The locations and composition of major Chinese naval units, according to the Pentagon. US Defense Department

Conventionally powered subs are the “more important part of their submarine force,” Clark said, particularly ones that can launch anti-ship missiles and those that use air-independent propulsion, or AIP, which allows nonnuclear subs to operate without access to atmospheric oxygen, replacing or augmenting diesel-electric systems.

Since the mid-1990s, China has built 13 Song-class diesel-electric attack subs and bought 12 Russian-made Kilo-class subs — eight of which can fire anti-ship cruise missiles.

Kilos are conventional diesel subs, which means they need to surface periodically.

“Even with that, they’re a good, sturdy, reliable submarine that carries long-range anti-ship missiles,” Clark said. On a shorter operation where a Kilo-class sub “can avoid snorkeling, it could … sneak up on you with a long-range attack, so that’s a concern for the US.”

China has also built 17 Yuan-class diesel-electric, air-independent-powered attack subs over the past two decades, a total expected to rise to 20 by 2020, according to the Pentagon.

“The Yuan AIP submarine is very good,” said Clark, a former US Navy submarine officer and strategist.

“For the duration of a deployment that it might normally take, which is two or three weeks, where it can stay on its AIP plant and never have to come up and snorkel, they’re very good,” Clark added. “That’s a big concern, I think, for US and Japanese policymakers.”

Yuan-class boats can threaten surface forces with both torpedoes and anti-ship missiles.

For US anti-submarine-warfare practitioners in the western Pacific, Clark said, “it’s the Yuan they generally point to as being their target of concern, because it does offer this ability to attack US ships and [is] hard to track and there may be few opportunities to engage it.”

Despite concerns China’s current diesel-electric subs inspire, they have liabilities.

As quiet as they are, they are still not as quiet as a US nuclear-powered submarine operating in its quietest mode. They don’t have the same endurance as US subs and need to surface periodically. China’s sub crews also lack the depth of experience of their American counterparts.

“Chinese submarines are not … as good as the US submarines, by far,” Clark said.

China’s subs have made excursions into the Indian Ocean and done anti-piracy operations in waters off East Africa, but they mostly operate around the first island chain, which refers to major islands west of the East Asian mainland and encompasses the East and South China Seas.

Chinese subs also venture into the Philippine Sea, where they could strike at US ships, Clark said.

Much of the first island chain is within range of Chinese land-based planes and missiles, which are linchpins in Beijing’s anti-access/area denial strategy. It’s in that area where the US and its partners could see their advantages thwarted.

The approximate boundaries of the first and second island chains in the western Pacific. US Defense Department

“Now the Chinese have the advantage of numbers, because they have a large number of submarines that can operate, and they’ve only got a small area in which they need to conduct operations,” Clark said.

China could “flood the zone” with subs good enough to “maybe overwhelm US and Japanese [anti-submarine warfare] capabilities.”

The anti-submarine-warfare capabilities of the US and its partners may also be constrained.

US subs would likely be tasked with a range of missions, like land attacks or surveillance, rather than focusing on attacking Chinese subs, leaving much of the submarine-hunting to surface and air forces — exposing them to Chinese planes and missiles.

“The stuff we use for ASW is the stuff that’s most vulnerable to the Chinese anti-access approach, and you’re doing it close proximity to China, so you could get stuck and not be able to engage their submarines before they get out,” Clark said.

Numbers and location also give China a potential edge in a “gray-zone” conflict, or a confrontation that stops short of open combat, for which US Navy leadership has said the service needs to prepare.

China’s subs present “a challenge [US officials] see as, ‘What if we get into one of these gray-zone confrontations with China, and China decides to start sortieing their submarines through the first island chain and get them out to open ocean a little bit so they’re harder to contain,'” Clark said.

“If we’re in a gray-zone situation, we can’t just shoot them, and we don’t necessarily have the capacity to track all of them, so now you’ve got these unlocated Yuans roaming around the Philippine Sea, then you may end up with a situation where if you decide to try to escalate, you’ve got worry about these Yuans and their ability to launch cruise missiles at your ships,” Clark added.

“As the home team, essentially, China’s got the ability to control the tempo and the intensity,” he said.

Beijing often deploys its coast guard to enforce its expansive maritime claims in the South China Sea (which an international court has rejected) and has built artificial islands containing military outposts to bolster its position.

When those coast guard ships encounter US Navy ships, China points to the US as the aggressor.

In the waters off the Chinese coast and around those man-made islands, “they do a lot of that because they’re on their home turf and protected by their land-based missiles and sensors,” Clark said. “Because of that, they can sort of ramp [the intensity] up and ramp it down … as they desire.”

The circumstances of a potential conflict may give Chinese subs an edge, but it won’t change their technical capability, the shortcomings of which may be revealed in a protracted fight.

“Can the Chinese submarines — like the Yuans that have limited time on their AIP plants — can they do something before they start to run out of propellant, oxygen, and start having to snorkel?” Clark said.

“So there’s a little bit of a time dimension to it,” he added. “If the US and Japan can wait out the Chinese, then their Yuans have to start snorkeling or pulling into port … that might make them more vulnerable.”